Since the early 1900's, it has been common to superimpose a radio frequency signal on electric power transmission lines and use that signal for control and communication functions at both ends of the line. Prominent among those functions is the sending of information on current and voltage levels, thus allowing protective relays to judge whether or not to cause circuit breakers to open and take the line out of service.
The radio frequency signal is generally applied between two phases of a three-phase line or between poles on a high voltage dc line. With widely separated conductors of that sort, some of the resulting radio-frequency field is coupled to the earth which, because of its high resistance, causes a gradual loss in radio-frequency signal strength, thus requiring periodic “repeater” stations, i.e. stations which take a weak signal, amplify it, and re-apply it to the line.
For transmission lines of very high voltage and considerable length, the need for many repeater stations poses an economic burden to the power line carrier communication option. Furthermore, the fact that inter-phase or inter-pole transmission requires continuity in two or more conductors makes it difficult to sustain a signal with one conductor out of service. Finally, a transmission medium with widely spaced conductors is quite vulnerable to electrical noise, e.g. the “static” caused by corona or electrical impulses resulting from lightning strokes in the vicinity of the line. All of the foregoing limitations are addressed by the invention.
FIG. 1 shows the configuration of a conventional high voltage three-phase alternating current (ac) power line in which a power-carrying cable (conductor) 1 is suspended by a chain of insulators 4 from a supporting structure 3. It is common to install “shield wires” 2, directly tied to the structure 3 and thereby to ground, to intercept lightning strokes. FIG. 2 shows a similar transmission line configured for high voltage bipolar dc power transmission in which just two cables are required; one for the positive pole and another for the negative. FIG. 3 shows a less common monopole dc power transmission configuration in which current is caused to flow on one cable and return through the earth.
Power transmission lines similar to that shown in FIG. 1 commonly operate at either 50 Hz or 60 Hz; those in FIGS. 2 and 3 at constant (dc) voltage. It is common practice to superimpose radio-frequency (carrier current) signals on the same conductor system so as to provide a means of communication from one end of the line to the other; e.g. communication of current and voltage information necessary to determine when a circuit breaker should disconnect the line from service, e.g. in the event of a short circuit within the line.
The power line carrier signal is normally in the frequency range from 30 to 500 kHz. It is commonly coupled to a three-phase transmission line as illustrated in FIG. 4. In this illustrative example the conductor 1 used on each of three phases of an alternating current line, a, b, and c, are connected to the station bus 7 through circuit breakers 6. The circuit breaker symbol is omitted in subsequent diagrams since it is not germane to the invention. In the example configuration of FIG. 4, wave traps 5 are interposed in phases a and b. These wave traps 5 are tuned to be resonant at the power line carrier radio frequency and therefore to effectively isolate the conductor 1 from the bus 7 at that frequency. The wave traps 5 represent negligible impedance to 50 Hz, 60 Hz or direct current flow.
In FIG. 4 the power line carrier transmitter/receiver 10 is connected to power conductors 1 for phases a and b by means of coupling capacitors 8 which represent a very low impedance to the carrier frequencies but very high impedance to power frequencies or to dc. Reactors (drain coils in this example) 9 present a very high impedance to carrier frequency and therefore allow the radio-frequency signal emanating from the transmitter/receiver 10 to be applied between cables 1 representing a and c phases of the three-phase power line or, conversely the incoming radio frequency signal on cables 1 of phases a and c from the remote line end to be received by the transmitter/receiver 10.
FIG. 5 shows the same coupling principles as FIG. 4 except applied to both positive and negative poles of a bipolar dc transmission line. In this case the power-carrying conductors 1 emanate from the high voltage dc inverter station.
Radio frequency voltage applied to an open conductor system such as shown in FIGS. 1, 2, and 3 create an electrostatic and electromagnetic field around the conductors used for that purpose. A portion of these fields extends to the earth below the transmission line. Unlike conductors 1 which, being made from aluminum, have very low electrical losses, the earth has high electrical resistance. The existence of radio frequency fields within the earth causes high losses. Thus the radio-frequency signal attenuates as it goes down the line. In order that a sufficiently strong signal be received at the remote end of the line, “repeater” stations, an example of which is shown in FIG. 6, are required every 50 miles or so. The repeater station simply transfers the weak radio-frequency signal from the conductor 1 on the left side of the diagram to a repeater device 16 by means of a coupling capacitor 8 and drain coil 9 in a manner previously described. The weak signal is then amplified by the repeater and re-applied to the conductor 1 by a second coupling capacitor 8 and drain coil 9. The wave trap 5 inserts no impedance to power frequency or dc current but isolates the left line section from the right insofar as radio frequency signals are concerned.
In understanding this invention it is useful to consider attenuation and other attributes of radio frequency applied to open-wire conductor systems in somewhat more detail. FIG. 7a shows a radio frequency signal being applied by a sine-wave source connected between two conductors separated by distance “s” and located a distance “H” above the earth. Because of their wide separation, a significant portion of the electric and magnetic field is within the earth, leading to relatively high energy loss and high attenuation. Two other properties of this configuration are important. Widely-spaced conductors also cause the applied radio frequency voltage to be propagated to points far from the line, thus requiring that the applied signal be relatively weak to meet propagation standards. Conversely, externally-generated noise, e.g. static generated by corona on either or both conductors or, on a larger scale, electrical impulses caused by lighting discharges in the vicinity of the line, will superimpose itself on whatever signal is received between widely spaced conductors, thus interfering with the received signal-potentially causing errors in data received. Corona is the result of small and local electrical discharges on the conductor's surface common with high voltage power lines.
FIG. 7b shows a sine-wave radio-frequency signal being applied between two conductors, now in close proximity. In this case the electric field is much more closely contained; the earth's affect on attenuation much less, as are both the external influence of the applied radio frequency and the coupling of noise from external sources.
FIG. 7c shows a hypothetical coaxial configuration for illustrative purposes only, in which the sine-wave radio frequency signal is applied between a center conductor and a concentric surrounding conductor. In this case the earth's poor resistance will have no effect on attenuation, no signal is radiated from the system, and it is invulnerable to externally generated noise.
Many high voltage ac and dc transmission lines use a cluster or bundle of closely-spaced cables for each phase or each pole. FIG. 8 (prior art) shows a bundle comprised of four conductors. A spacer is used to separate each conductor from the others by a distance of about 18 inches. These spacers are positioned every 100 feet or so. There are many designs for such spacers. In FIG. 8, a simplified example, the spacer consists of four rigid spacer bars 14, which hold the primary conductors 11 in place by means of clamps 13. Spacers such as that shown in FIG. 8 hold conductors apart mechanically but normally connect them together electrically.
Four-conductor bundles such as shown in FIG. 8 are common at the highest of today's ac and dc voltages, e.g. 765,000 volts ac and 600,000 volts dc. At higher voltages now in planning and/or construction, as many as eight conductors per bundle are being considered. Subdividing the aluminum represented by each phase or pole into more subconductors reduces corona and the associated electrical and audible noise.
To date, intra-bundle power line carrier transmission has been proposed by transmission of signals between various of the subconductors within a bundle while leaving those conductors arrayed in a circular position and insulating them from one-another.